To meet the demand for wireless data traffic having increased since deployment of 4G (4th-Generation) communication systems, efforts have been made to develop an improved 5G (5th-Generation) or pre-5G communication system. Therefore, the 5G or pre-5G communication system is also called a ‘beyond 4G network’ or a ‘post LTE system’.
The 5G communication system is considered to be implemented in higher frequency (mmWave) bands, e.g., 60 GHz bands, so as to accomplish higher data rates. To decrease propagation loss of the radio waves and increase the transmission distance, the beamforming, massive multiple-input multiple-output (MIMO), full dimensional MIMO (FD-MIMO), array antenna, an analog beam forming, large scale antenna techniques are discussed in 5G communication systems.
In addition, in 5G communication systems, development for system network improvement is under way based on advanced small cells, cloud radio access networks (RANs), ultra-dense networks, device-to-device (D2D) communication, wireless backhaul, moving network, cooperative communication, coordinated multi-points (CoMP), reception-end interference cancellation and the like.
In the 5G system, hybrid FSK and QAM modulation (FQAM) and sliding window superposition coding (SWSC) as an advanced coding modulation (ACM), and filter bank multi carrier (FBMC), non-orthogonal multiple access (NOMA), and sparse code multiple access (SCMA) as an advanced access technology have been developed.
Mobile communication technology faces fierce challenges due to the increasing demand of the internet and the internet of things (IoT). According to a report of international telecommunication union (ITU), ITU-R M.[IMT.BEYOND 2020.TRAFFIC], the mobile traffic volume in 2020 is estimated to be almost 1000 times of that in 2010 (e.g., in the 4G era), and the number of connected user terminals may exceed 17 billion. The number of connected devices will see more drastic growth when a mass of IoT devices are gradually connected to the mobile communication network. In view of the challenge, the fifth generation mobile communication technology (5G) for the 2020 era is being widely studied by the communication industry and the academia. The ITU report ITU-R M.[IMT.VISION] discusses the framework and overall target of 5G, detailing the prospect of demands for 5G, application scenarios, and key parameters. The ITU report ITU-R M.[IMT.FUTURE TECHNOLOGY TRENDS] provides information regarding future trends of the 5G technology, aiming at remarkably increasing system throughput, providing uniform user experiences, improving extensibility to support IoT, reducing time delay, increasing power efficiency, reducing costs, increasing network flexibility, supporting emerging services, improving flexibility in utilizing the spectrum resources and the like.
Modulation waveforms and multiple access techniques are important basis for air-interface design in all of wireless communication systems, and 5G is no exception. Orthogonal frequency division multiplexing (OFDM) is a typical multi-carrier modulation (MCM) technique. At present, OFDM has been widely used in audio/video broadcasting systems and civil communication systems, e.g., long term evolution (LTE) systems of 3GPP, digital video broadcasting (DVB) and digital audio broadcasting (DAB) in Europe, very-high-bit-rate digital subscriber loop (VDSL), IEEE802.11a/g Wireless Local Area (WLAN), IEEE802.22 wireless regional area network (WRAN), IEEE802.16 world interoperability for microwave access (WiMAX), and so on. OFDM divides a broad band channel into multiple parallel narrow band subchannels/subcarriers to convert transmission of a high rate data flow in a frequency selective channel into transmission of multiple lower rate data flows in multiple parallel flat subchannels. OFDM can greatly improve the anti-multipath interference capabilities of the system. Furthermore, modulation and de-modulation of OFDM can be simplified using inverse fast fourier transform/fast fourier transform (IFFT/FFT). In addition, the use of cyclic prefix (CP) converts the linear convolution of a channel into circular convolution. According to characteristics of circular convolution, when the length of CP is larger than the maximum multipath time delay in the channel, inter-symbol interference (ISI) can be eliminated simply by using single tap frequency domain channel equalization. The processing complexity of receivers is remarkably reduced. CP-OFDM can generate modulation waveforms satisfying the demands of 4G mobile broadband (MBB) services, but may have insufficiencies in 5G scenarios. For example, the CP for anti-ISI may significantly reduce the spectrum efficiency in 5G low time delay scenarios. Because low time delay transmission may greatly reduce the length of OFDM symbols while the length of CP is decided only by the channel impulse response, the ratio of CP length to OFDM symbol length may become very large. The overhead will result in remarkable loss in spectrum efficiency, and it is unacceptable. For another example, strict requirement on time synchronization may cause large signaling overhead which is necessary to maintain close-loop synchronization in 5G IoT scenarios. Further, the strict time synchronization scheme may make frame structures less flexible and cannot satisfy different synchronization demands of different types of services. For still another example, the rectangular pulse employed by OFDM may result in large out-of-band leakage because the side lobes of the waveform roll off very slowly, which is also the reason why OFDM is very sensitive to carrier frequency offset (CFO). 5G may be required to provide access/share of spectrum fragments more flexibly, but the out-of-band leakage of OFDM will greatly limit the flexibility of spectrum access, i.e., a large number of frequency domain guard band is needed. Therefore, the spectrum efficiency is reduced. The above insufficiencies are mainly resulted from intrinsic characteristics of OFDM. Although proper measures may be adopted to mitigate the effect of the insufficiencies, the measures will increase the system design complexity, and the problems cannot be solved fundamentally.
Therefore, according to an ITU report ITU-R M. [IMT.FUTURE TECHNOLOGY TRENDS], a few new waveform modulation techniques (based on MCM) are proposed for 5G. Among the new waveform modulation techniques, filter bank multiple carrier (FBMC) has become one of hot topics being studied. Since FBMC allows flexibility in design of prototype filters, filters having good time/frequency localization (TFL) can be used for shaping transmission waveform to generate transmission signals with good characteristics, e.g., not needing CP for anti-ISI which improves spectrum efficiency, low out-of-band leakage which allows flexibility in providing access to spectrum fragments and insensitivity to CFO. Typical FBMC systems usually adopt an offset quadrature amplitude modulation (OQAM) technique to maximize spectrum efficiency, thus are generally referred to as FBMC/OQAM systems or OFDM/OQAM systems.
FBMC has some good characteristics that OFDM does not have, thus gains attention in 5G research. FBMC, however, also faces challenges when applied to wireless communication systems due to some intrinsic deficiencies, which are being studied. One of the problems to be solved is a long head and tail effect of time domain waveforms generated by filters used by FBMC, which is also referred to as transition period problem. In uplink transmission of short data bursts, if the length of data bursts are extended to cover the tail so as to avoid overlapping of the tail with the head of other data bursts, fewer symbols will be transmitted within effective time and spectrum efficiency will be reduced. Thus, FBMC is regarded to be only suitable for transmission of long data bursts. If the length of data bursts does not cover the tail, the tail may overlap with the head of other data bursts. If the overlapping problem is not solved properly, there will be severe interference, which also reduces the spectrum efficiency. Besides the inter-user interference, in a time division duplexing (TDD) system, the uplink/downlink switch time should also be increased to avoid uplink/downlink interference, but this will also reduce the spectrum efficiency. In addition, the head and tail effect may also bring challenges for system flexibility and system performance in aspects such as frequency hopping, asynchronous transmission. A solution to the problem is to simply cut off the tail to avoid the tail overlapping with the head of other data bursts. But truncating the waveform may distort the signals, which also reduces the spectrum efficiency. In addition, truncating the waveform may extend the waveform in the frequency domain and increase inter-carrier interference (ICI). Thus, signal truncation is not an effective measure.
In view of the foregoing, deficiencies of FBMC may be dealt with to strengthen the competitiveness of FBMC as a candidate technique for 5G besides exploiting its advantages. For services based on sporadic access in 5G networks, especially in the IoT scenarios, it is necessary to find an effective solution to the problems resulted from the head and tail effect.